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An international team of professional and amateur astronomers, using simple off-the-shelf equipment to trawl the skies for planets outside our solar system, has hauled in its first "catch."

This artist's impression shows a dramatic close-up of the extrasolar planet XO-1b passing in front of a Sun-like star 600 light-years from Earth. The Jupiter-sized planet is in a tight four-day orbit around the star. Credit: NASA, ESA and G. Bacon (STScI)

The astronomers discovered a Jupiter-sized planet orbiting a Sun-like
star 600 light-years from Earth in the constellation Corona Borealis.
The team, led by Peter McCullough of the Space Telescope Science
Institute in Baltimore, Md., includes four amateur astronomers from
North America and Europe.

Using modest telescopes to search for extrasolar planets allows for a
productive collaboration between professional and amateur astronomers
that could accelerate the planet quest.

"This discovery suggests that a fleet of modest telescopes and the help
of amateur astronomers can search for transiting extrasolar planets many
times faster than we are now," McCullough said. The finding has been
accepted for publication in the Astrophysical Journal.

McCullough deployed a relatively inexpensive telescope made from
commercial equipment to scan the skies for extrasolar planets. Called
the XO telescope, it consists of two 200-millimeter telephoto camera
lenses and looks like a pair of binoculars. The telescope is on the
summit of the Haleakala volcano, in Hawaii.

"To replicate the XO prototype telescope would cost $60,000," McCullough
explained. "We have spent far more than that on software, in particular
on designing and operating the system and extracting this planet from
the data."

McCullough's team found the planet, dubbed X0-1b, by noticing slight
dips in the star's light output when the planet passed in front of the
star, called a transit. The light from the star, called XO-1, dips by
approximately 2 percent when the planet XO-1b passes in front of it.
The observation also revealed that X0-1b is in a tight four-day orbit
around its parent star.

Although astronomers have detected more than 180 extrasolar planets,
X0-1b is only the tenth planet discovered using the transit method. It
is the second planet found using telephoto lenses. The first, dubbed
TrES-1, was reported in 2004. The transit method allows astronomers to
determine a planet's mass and size. Astronomers use this information to
deduce the planet's characteristics, such as its density.

The team confirmed the planet's existence by using the Harlan J. Smith
Telescope and the Hobby-Eberly Telescope at the University of Texasís
McDonald Observatory to measure the slight wobble induced by the planet
on its parent star. This so-called radial-velocity method allowed the
team to calculate a precise mass for the planet, which is slightly less
than that of Jupiter (about 0.9 Jupiter masses). The planet also is much
larger than its mass would suggest. "Of the planets that pass in front
of their stars, XO-1b is the most similar to Jupiter yet known, and the
star XO-1 is the most similar to the Sun," McCullough said, although he
was quick to add, "but XO-1b is much, much closer to its star than
Jupiter is to the Sun."

The astronomer's innovative technique of using relatively inexpensive
telescopes to look for eclipsing planets favors finding planets orbiting
close to their parent stars. The planet also must be large enough to
produce a measurable dip in starlight.

The planet is the first discovered in McCullough's three-year search for
transiting extrasolar planets. The planet quest is underwritten by a
grant from NASA's Origins program.

McCullough's planet-finding technique involves nightly sweeps of the sky
using the XO telescope in Hawaii to note the brightness of the stars it
encounters. A computer software program sifts through many thousands
of stars every two months looking for tiny dips in the stars' light,
the signature of a possible planetary transit. The computer comes up
with a few hundred possibilities. From those candidates, McCullough and
his team select a few dozen promising leads. He passes these stars on to
the four amateur astronomers to study the possible transits more
carefully.

From September 2003 to September 2005, the XO telescope observed tens of
thousands of bright stars. In that time, his team of amateur astronomers
studied a few dozen promising candidate stars identified by McCullough
and his team. The star X0-1 was pegged as a promising candidate in June
2005. The amateur astronomers observed it in June and July 2005,
confirming that a planet-sized object was eclipsing the star.
McCullough's team then turned to the McDonald Observatory in Texas to
obtain the object's mass and verify it as a planet. He received the news
of the telescope's observation at 12:06 a.m. Feb. 16, 2006, from Chris
Johns-Krull, a friend and colleague at Rice University.

"It was a wonderful feeling because the team had worked for three years
to find this one planet," McCullough explained. "The discovery
represents a few bytes out of nearly a terabyte of data: It's like
trying to distill gold out of seawater."

The discovery also has special familial significance for the astronomer.
"My father's mentor was Harlan J. Smith, the man whose ambition and hard
work produced the telescope that we used to acquire the verifying data."

McCullough believes the newly found planet is a perfect candidate for
study by the Hubble and Spitzer space telescopes. Hubble can measure
precisely the starís distance and the planet's size. Spitzer can
actually see the infrared radiation from the planet. By timing the
disappearance of the planet behind the star, Spitzer also can measure
the "ellipticity," or "out-of-roundness," of the planet's orbit. If the
orbit is elliptical, then the varying gravitational force would result
in extra heating of the planet, expanding its atmosphere and perhaps
explaining why the object's diameter seems especially large for a body
of its calculated mass.

"By timing the planet's passages across the star, both amateur and
professional astronomers might be lucky enough to detect the presence of
another planet in the XO-1 system by its gravitational tugs on XO-1b,"
McCullough said. "It's even possible that such a planet could be similar
to Earth."

The Space Telescope Science Institute in Baltimore is operated for NASA
by the Association of Universities for Research in Astronomy, Inc.,
Washington.